Chromatographic and spectroscopic methods contribute to the detection of increasingly more and more fragrant substances used for aromatizing foods and semi-luxuries such as tobacco, and odiferous substances utilized in the cosmetic industry. The chemical structure of these substances is determined and their synthetic production follows if required. A major part of the research is done on fragrant components of various kinds of tobacco. A review of a large number of such tobacco components can be found in a paper by C. R. Enzell, J. Wahlberg, A. J. Aasen, in Fortschritte der Chemie Organischer Naturstoffe, vol. 34. Springer-Verlag, Vienna and New York (1977) p. 1-79. An important sensor-active group of those natural fragrant tobacco components are carotenoid derivatives. Within this group, a key compound is 3-oxo-.alpha.-ionol (formula 1 below), isolated from tobacco by A. J. Aasen, B. Kimland and C. R. Enzel (Acta Chem. Scand., 25, 1481-1482 (1971)), due to the fact that the compound can be used as a starting material to obtain other derivatives (compare Enzell et al. referred to hereinabove). Such derivatives are, among others, dehydration products resulting from ( 1) and designated by numerals (2) thru (6), which have also been detected in tobacco (compare A. J. Aasen, B. Kimland, S.-O. Almquist and C. R. Enzell, Acta Chem. Scand., 26, 2573-2576 (1972), the publication being referred to as Reference A hereinafter). A term "megastigmatrienones" was proposed by the above authors to name these products. ##STR1##
The megastigmatrienones (3) thru (6) have also been detected in the "kudzu" (Puerania lobata) essential oil by S. Shibata et al. (J. Agric. Biol. Chem., 42, 195-197 (1978)) and as an ingredient of liquid components of "kukoshi" berries (Lycium chinense M.) by A. Sannai et al. (J. Agric. Biol. Chem., 47, 2397-2399 (1983)).
The first synthesis of the megastigmatrienones (3) thru (6) was described as early as 1963 (U.S. Pat. No. 3,211,157 to R. L. Rowland), i.e. long before these substances were known to be tobacco components (see Reference A). The synthesis involved the reduction of dehydro-.alpha.-ionone (7) with sodium borohydride to alcohol (8) which was then converted into an allyl alcohol (9) using 2N sulfuric acid. The allyl alcohol (9) was then oxidized, using manganese dioxide, to give the megastigmatrienones (3) thru (6). ##STR2##
E. Demole and P. Enggist (Helv. Chim. Acta, 57, 2087-2089 (1974)) describe a synthesis starting from a ketoketal (10) obtained from oxo-isophoron. The ketoketal (10) was subjected to exchange reactions with dianion of but-3-in-2-ol and subsequently with lithium aluminium hybride to yield a mixture of megastigmatrienones (3) thru (6) with various hydroxy ketones of megastigmic type (11) thru (14). ##STR3##
S. Torii, T. Inskuchi and H. Ogawa (Bull. Chem. Soc. Japan, 52, 1233-1234 (1979)) started from dimedone (15). They obtained enolized diketones (16) by aldol condensation of the enolate with crotonaldehyde and subsequent dehydration. The diketones were subjected to exchange reaction with methylithium and megastigmatrienones (3) thru (6) were obtained.
B. M. Trost and J. L. Stanton (J. Amer. Chem. Soc., 97, 4018-4025 (1975)) reduced 3,4-dehydro-.beta.-ionone (17), and obtained, through subsequent dehydration and sulfenylation, a compound (18) which was converted into the megastigmatrienones (3) thru (6) by way of a sulfenate-sulfoxide rearrangement.
O. Takazwawa, H. Tamura, K. Kogami and K. Hayashi (Bull. Chem. Soc. Japan, 55, 1907-1911 (1982)) described a synthesis of the megastigmatrienones (3) to (6) using isophorone (19) as a starting compound. Isophorone trimethylsilyl enol ether (20 a) was condensed with crotonaldehyde to give oxo-damascol (20 b). The latter was dehydrated, resulting in a mixture of megastigmatrienones (3) to (6). ##STR4##
The acid-catalyzed dehydration of 3-oxo-.alpha.-ionol (1) is known so far to have been described only once in technical literature. Aasen, Kimland, Almquist and Enzell (see Reference A) heated 3-oxo-.alpha.-ionol (1) together with potassium bisulfate at 100.degree. C., solvent-free, or with p-toluene sulfonic acid (benzene as solvent, under reflux) for 2 to 3 hours. As a result, a mixture of isomeric megastigmatrienones (2) to (6) was obtained at all times. The proportion of the respective isomers in the mixture was determined using gas chromatography to be EQU (2):(3):(4):(5):(6)=10:1:10:1:10
According to Reference A, a similar mixture of final compounds can be obtained under the above conditions starting from an acetate prepared from compound (1). However, experimental details have not been disclosed.